Bicomponent fibers are fibers produced by extruding two polymers from the same spinneret with both polymers contained within the same filament. The advantage of the bicomponent fibers is that it possesses capabilities that can not be found in either of the polymers alone. Depending on the arrangement and relative quantities of the two polymers, the structure of bicomponent fibers can be classified as core and sheath, side by side, tipped, microdenier, mixed fibers, etc.
Sheath-core bicomponent fibers are those fibers where one of the components (core) is fully surrounded by the second component (sheath). The core can be concentric or eccentric relative to the sheath and possessing the same or different shape compared to the sheath. Adhesion between the core and sheath is not always essential for fiber integrity. The sheath-core structure is employed when it is desirable for the surface of the fiber to have the property of the sheath such as luster, dyeability or stability, while the core may contribute to strength, reduced cost and the like. A highly contoured interface between sheath and core can lead to mechanical interlocking that may be desirable in the absence of good adhesion.
Generally, composite bicomponent sheath-core fibers have been used in the manufacture of non-woven webs, wherein a subsequent heat and pressure treatment to the non-woven web causes point-to-point bonding of the sheath components, which is of a lower melting point than the core, within the web matrix to enhance strength or other such desirable properties in the finished web or fabric product.
Poor abrasion resistance of Polyethylene/Polyethylene Terephthalate (PE/PET) sheath/core bicomponent spunbond has been an industry recognized problem since the last 10-15 years. Various approaches have been devised attempting to solve this problem. Similar problems also affect many other frequently used sheath/core structures such as PE/Polyesters (for example, Polybutylene Terephthalate (PBT), Polytrimethylene Terephthalate (PTT), Polylactide (PLA)), PE/Polyolefins, PE/Polyamide, PE/Polyurethanes.
A first method is directed to the modification of fiber structure to improve adhesion between the sheath and core component. For example, a mixture of EVA (ethyl vinyl acetate) and PE was suggested for a sheath component in U.S. Pat. Nos. 4,234,655, 5,372,885 teaches the use of a blend of maleic anhydride grafted HDPE and un-grafted LLDPE (linear low density polyethylene). A mixture of PE and acrylic acid copolymer was suggested in U.S. Pat. No. 5,277,974 and a blend of HDPE (high density polyethylene) with LLDPE was claimed in WO 2004/003278A1 as a sheath component.
An approach for improving abrasion resistance proposed is by increasing the bond area of the spunbond, for example, U.S. Pat. Appl. Publ. No. 20020144384 teaches a non-woven fabric with a bond area of at least about 16%, 20% or 24%. However, higher bond area samples results in loss of softness and drapeability of bicomponent spunbond, which is not desirable for many applications especially for medical apparel such as surgical gowns. At the other extreme, nonwovens with small bond areas tend to make soft feeling but very weak fabric.
Another approach involves the use of a number of treatments, such as multiple washings and chemical treatments.
Yet another approach, which is of particular relevance to the subject matter of this application, is directed to adopting a specific thermal bonding pattern for nonwoven fabric comprising a pattern having an element aspect ratio between about 2 and about 20 and unbonded fiber aspect ratio of between about 3 and about 10, as disclosed in U.S. Pat. No. 5,964,742. Such a pattern has been found to possess a higher abrasion resistance and strength than a similar fabric bonded with different bond patterns of similar bond area.
There remains a need for a nonwoven fabric without resort to chemical treatments having good bonding strength (i.e. tensile strength and abrasion resistance) yet also having good fabric softness, particularly at relatively high bonding area.
Accordingly, it is an object of this invention to provide a nonwoven fabric with a high bonding area while retaining softness and comparable or better tensile strength and abrasion resistance compared to fabrics bonded with other known patterns.
It is another object of this invention to provide a method of preparing a nonwoven fabric with a high bonding area while retaining softness and comparable or better tensile strength and abrasion resistance.